Systems for measuring variable parameters are often expensive and bulky. Additionally, they often rely upon a power source, usually in the form of batteries, which may need frequent recharging in order to keep the circuits powered. Where the devices need to be small, for example where tyre pressures are monitored, such systems may be difficult to access, thereby making changing of the power source a potentially expensive and difficult task; disposal and safety issues relating to batteries can also be a major issue.
Radio frequency identification (RFID) tags and readers are used in shops for reducing theft. The tags are attached to products and readers are located by the doors to the shop, with the tags triggering an alarm should they not be deactivated before passing the reader.
Such tags contain no power source and rely upon ‘activation’ via electromagnetic induction from the reader in order to power the circuit contained within the tags.
The operating frequencies, also referred to as their nominal centre frequency or an associated sideband frequency, are, generally, not tightly controlled, as the readers can accommodate variations in operating frequencies. Traditionally, the reader may rely upon a range of frequencies in order to locate the tag.